The invention relates to a tire with radial carcass reinforcement anchored within each bead to at least one bead wire, and comprising a crown reinforcement formed by at least two so-called working plies, which are superposed and formed of cords or cables which are parallel to each other in each ply and crossed from one ply to the next, forming angles of at most 45xc2x0 in absolute value with the circumferential direction of the tire.
Application FR 2,744,955, in order to reduce the operating temperature of a tire with radial carcass reinforcement of the xe2x80x9cheavy vehiclexe2x80x9d type, while having a solution which is as effective as it is economic, advocates adding to said carcass reinforcement a crown reinforcement comprising at least two working crown plies of inextensible cables, crossed from one ply to the next, forming angles of between 10xc2x0 and 45xc2x0 in absolute value with the circumferential direction, characterized in that, in the absence of any ply formed of inextensible cables forming an angle greater than 45xc2x0 with the circumferential direction, an additional, axially continuous ply, formed of metal elements oriented substantially parallel to the circumferential direction, is placed radially between the working plies, said additional ply having an axial width at least equal to 50% of the maximum axial width S0 of the carcass reinforcement, and at least equal to 1.05 times the axial width of the widest working crown ply.
In the above context, the additional ply may be formed of so-called semi-elastic continuous steel cables, that is to say cables having relative elongations at break of between 2% and 4%. These cables make it possible to obtain the level of rigidity suitable for proper distribution of the circumferential tension between the working crown plies and the additional ply. Said cables are advantageously said to be xe2x80x9cbimodularxe2x80x9d, that is to say, having a curve of tensile stress as a function of relative elongation having gradual slopes for the low elongations and a substantially constant, steep slope for the higher elongations. The very low modulus before curing for elongations of less than 2% permits an increase in the circumferential development of the additional ply during the curing of the tire.
The additional ply may also be formed of metal cables made of steel, oriented circumferentially and cut so as to form sections of a length very much less than the circumferential length of the ply, the cuts between sections being axially offset relative to each other. Such an embodiment makes it possible, in simple manner, to impart the desired rigidity to the additional ply, whatever it may be.
The selection of elastic cables or cut cables for reinforcing the additional ply does not permit the best fatigue strength of said ply, either as a result of a reduction in the breaking load of the elastic cables, or as a result of the existence of stress concentrations in the calendering mix of the cut cables.
In order to impart better fatigue strength of the additional ply of circumferential metallic reinforcement elements, while permitting easier industrialization, the invention proposes the use, as reinforcement elements, of metallic elements which undulate in the plane of the ply. Thus the tire with radial carcass reinforcement according to the invention, having a crown reinforcement comprising at least two working crown plies of inextensible metallic reinforcement elements, crossed from one ply to the next, forming angles of between 10xc2x0 and 45xc2x0 with the circumferential direction, is characterized in that, in the absence of any ply formed of inextensible reinforcement elements forming an angle greater than 45xc2x0 with the circumferential direction, an additional, axially continuous, ply formed of undulating reinforcement elements is placed radially between the working plies, said ply having an axial width equal to at least 50% of the maximum axial width of the carcass reinforcement, the undulations of the reinforcement elements being parallel to each other, in phase and oriented parallel to the circumferential direction, and the ratio of the rigidity per unit length of the additional ply to the total of the rigidities per unit length of the other plies of the crown reinforcement being at most 0.10.
The rigidity of extension per unit length of a ply of reinforcement elements (measured on a ply of elements which is taken from the vulcanized tire) results from the tensile force exerted in the average direction of said elements per unit of width of ply which is necessary to obtain a given relative elongation xcex5, and may be expressed by the formula R=1/pxc2x7dF/dxcex5, R being the rigidity of the ply in question, p the pitch between elements of said ply, dF/dxcex5 the derivative of the tensile force per element relative to the relative elongation, and e being equal to 0.5%, or by the formula R=dF/dxcex5, dF/dxcex5 then being the derivative of the tensile force per unit of length of the ply relative to the relative elongation.
Preferably the ratio of the amplitude a to the wavelength xcex of the undulations is between 4% and 15%, the amplitude a of an undulation being, by definition, measured peak-to-peak. The ratio a/xcex of between 4% and 15% makes it possible to have reinforcement elements which do not impede the shaping of the tire blank in the vulcanization mold, while permitting the obtention, after curing, mounting and inflation of the tire, of a rigidity per unit length of the crown reinforcement which is necessary and sufficient to improve the endurance of said crown reinforcement.
The reinforcement elements which undulate in the plane of the ply are preferably metallic, and better still metallic cables made of steel, which provide the additional ply with a greater fatigue strength.
Whatever the material of which the reinforcement elements are formed (cables or monofilaments), they are advantageously of greater diameter than that of the reinforcement elements, generally metallic cables made of steel, the plies of the crown reinforcement which are located radially on either side of said additional ply.
Undulating metallic elements, the average direction of which is circumferential, are elements having undulations with axes having a direction forming an angle within the range of +5xc2x0,xe2x88x925xc2x0 around 0xc2x0 with the circumferential direction.
It is also advantageous for the additional ply to have an axial width at least equal to 1.05 times the axial width of the widest working crown ply.
One advantageous manner of using the additional ply of undulating circumferential elements, which is more particularly suitable for tires of H/S form ratio of at least 0.50, consists in imparting to the first working crown ply a meridian curvature substantially equal to the meridian curvature of the subjacent carcass reinforcement, so as to be able to arrange it parallel to said carcass reinforcement without the interposition of profiled members. The additional ply is then provided with a substantially zero curvature, being separated from the first working ply by suitable profiled members, of substantially triangular shape.
The crown reinforcement according to the invention will advantageously be finished off by a so-called protective crown ply, formed of straight elastic metal cables made of steel, oriented relative to the circumferential direction at an angle substantially equal to the angle formed by the cables of the radially outermost working crown ply, and the axial width of which is at least equal to the axial width of the radially outermost working ply.